Metallic articles can be protected against corrosion by covering them with a metallic, inorganic or organic protective coat. The organic protective coats, in particular, are provided with specific pigments and/or fillers in order to increase their corrosion protection capability. These additives include red lead, zinc chromate, zinc phosphate, talc, graphite and mica. Alternatively, it is possible to employ organic compounds as anti-corrosion pigments, alone or in combination with inorganic pigments and fillers. Examples of such organic compounds are benzidine phosphate, benzidine molybdate, benzidine hexacyanoferrate, organic phosphonic and arsonic acids and both aromatic and aliphatic carboxylic acids and their salts, such as benzoates and laurates.
Owing to their toxic and/or carcinogenic properties, the highly effective lead pigments and chromate pigments can no longer be used for corrosion protection. The pigments used to date in their place, zinc phosphate and zinc tetraborate, are of only low activity in comparison. Zinc salts initially require substrate corrosion, as for example, in the case of iron: EQU Fe.fwdarw.Fe.sup.2+ 2e.sup.- EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.-.fwdarw.2OH.sup.-
and are then intended to form basic complexes of low solubility with the OH.sup.- ions formed. These complexes are intended to either adhere firmly to the substrate surface or to be precipitated into faults in an anti-corrosion primer and so plug these faults. A prerequisite for this is, firstly, that the corresponding zinc salt is present in an adequate pigment volume concentration and has not meanwhile been leached out owing to its solubility in water, and, secondly, that there are no other complexing species in the coating or in the adjacent corrosive medium. As a result, the zinc salt pigments frequently fail, and/or are markedly inferior in their action to the classical active pigments red lead and zinc chromate.
It is known from DD 281 427 that metal phthalocyanines show very good results as anti-corrosion pigments in coating materials on iron. They exert their effect both as pure pigment and in combination with a conductive carrier.
An improved formulation is the subject of EP 0 675 173, consisting of metal phthalocyanine, a conductive component, a component which binds hydroxide ions, and platelet-shaped pigments. A disadvantage of this combination is that the platelet-shaped pigment causes the coating material to be highly porous. The consequence is that the water which contaminates the coating has unhindered access to the metal surface, thereby accelerating the corrosion.
There is therefore a need for lead- and chromate-free pigment formulations which can be used for primer coats on corrosion-susceptible metals and which develop an anti-corrosion action which is equal to the protective effect of lead and chromate pigments.